Aero engines are critical components in aviation, relying on thermodynamics to convert fuel energy into thrust. Maintenance of these engines constitutes a significant portion of operational costs. Gas path analysis (GPA) is essential for engine health management, relying on performance modeling to simulate engine operations accurately. This project outlines a comprehensive study aimed at enhancing gas path modeling, conducted in collaboration with KLM Engine Services. The objective was to create a highly accurate model for simulating aero engines, utilizing a wealth of operational data for improved accuracy. The study focuses on the GEnx-1b turbofan model, addressing flaws in the existing approach, particularly related to Reynolds effects. Novel "Off design (OD) functions" are introduced to enhance model accuracy, and a systematic methodology for model enhancement is presented. The newly corrected model was validated against operational flight data, demonstrating high accuracy in simulating engine performance, with promising implications for maintenance efficiency.